Monster Falls Insanity
by super cool username
Summary: Dipper and Mabel are glad to finally have a break from hectic college life, and are looking forward to a relaxing summer in Gravity Falls. Well, as relaxing as it can get in this crazy town, They've had a lot of weird summer before, but this one takes the cake! Older!Twins Monster Falls AU. Hope you like! Rated K for possible scenes of violence and language. Just being safe.
1. Chapter 1

It was a bright, sunny day. I little too sunny actually. It was sweltering hot, and business at the shack had been so slow that Stan had decided to close up shop (Although that might have been because the twins had just gotten back, and had just graduated college) and everyone went down to the lake to try and cool off. The lake had expanded a bit, since a beavers dam had broken recently on a rather unknown river. Eager to finally get some relief from the heat, Dipper and Mabel instantly jumped in, quickly followed by Wendy and Soos, and finally, after a bit of persuasion, Stan. The five were messing around and having a good time. That is, until a certain someone showed up.

Dipper frowned as the color was sucked out of their surroundings. "What do you want Bill?" He said, not bothering to turn around. "Aww, come on! I'm not allowed to see you guys!? It's been forever since I last saw you two! Can I at least get a 'Hi, how've you been?'" Bill said in a mock offended tone. "Hi! How've you been?" Mable called from across the water. "See, your sister's nice! Why can't you learn to be more like her?" Dipper scoffed.

"What are you really doing here, Bill?" Bill rolled his eye. "For once, nothing! I'm just bored!" Dipper huffed, before he noticed Mabel swimming up behind him. "Plus, like I said, you guys have been gone for like, forever, and-Hey, why are you grinning like that?" Dipper quickly straightened his face. "No reason." But he quickly dissolved into laughter as his sister splashed him from behind. "AH!" Bill whipped around, before glaring at Mabel.

"Oh, so that's how you wanna play, huh, Shooting Star! Fine!" At that, he sent a wave of water towards her. Mabel started laughing hysterically when she surfaced. Bill looked over, only to see Dipper still eyeing him suspiciously. "Yeesh kid, fine. I showed up because something big about about to happen. I can't tell what yet, but one thing for sure, I wanna be there when it happens. Anyways, nice seeing you guys, bye!" And with that ha vanished.

"Ha! Gotcha!" Wendy cried, dunking Dipper under the water. He let out a cry as he was submerged, and came up coughing and spluttering, and grabbed Wendy to pull her under with him again. Wendy came up with him and gasped. After she had caught her breath, she hi-fived Dipper, giving him a, "Nice one, dude!" Meanwhile, Soos had sneaked up on Mabel, or so he thought, until she turned around and tackled him.

They both came up laughing. "Hey, Stan hasn't gone under yet!" Dipper pointed out, grinning at his great uncle. "And I don't plan to! Try and catch me, suckers!" Stan cried, swimming out farther into the lake. After a good fifteen minutes of chasing him, (for an old guy, he could be fats when he wanted to!), they managed to catch him and push him under. The sun was starting to go down, and they decided that that was enough swimming for the day. They trudged home, chatting happily. (except for Stan, who was still pretending to be mad about being dunked.)

Dipper sighed, flopping down on his bed. What a day. "Hey, dip-dot?" Dipper turned over so he could see his sister. "Yeah? And don't call me that." Mabel just laughed. "Do you really think something big is gonna happen soon." Dipper frowned. "I don't know. But seeing as we're here now, it's more possible than before." At that, Mabel grinned. They'd had a joke that the only time supernatural stuff seemed to happen was when they were around. "Well, whatever happens, being tired won't help!" Mabel said flopping back against her pillow. "Yeah."

" 'Night, Bro-bro!"

"Good night, Mabel."

 **This is my first real writing project so constructive criticism is appreciated, just no flames please! ^^**

 **~ Cool**


	2. Chapter 2

"Wake up, bro-bro!"

Dipper groaned and shielded his eyes from the light streaming through the attic window. Despite the fact that the two were much older now, they had never really considered having their own rooms. They shared practically everything after all. They had found a way to accommodate the two of them after a bit of rearranging. "Alright, alright. I'm up." Dipper muttered, waving his sister away. He sat up and stretched, yawning. Mabel had already run out of the room, most likely to get breakfast. Walking over to his dresser, he quickly pulled on a flannel t-shirt and a pair of jeans.

Sure enough, Mabel was at the table, scarfing down pancakes like there was no tomorrow. Getting a glass of orange juice, Dipper sat and started eating at a considerably slower pace. "I'm gonna go hang out with my friends for the day bye!" Mabel said, attempting to run out the door. "Not so fast!" Sta caught her by the back of her sweater. (Dipper would never understand how she managed to wear those things in summer.) "I haven't forgotten about you slacking off last week!" Mabel pouted. "Awww, but Grunkle Stan-" "No buts! Now get to work!" Mabel slouched off, texting Candy and Grenda that she couldn't make it.

Stan walked into the living room and turned on the tv to the news channel, where the reporter had just started a new report. "In other news, the town mayor has decided to start using the lake as the town's main water source, in interest of cutting down expenses-" "Boring." Stan said, changing the channel. "What, aw come on, that looked interesting!" "Kid, anything you find interesting is bound to be boring, so you're not encouraging me to keep watching." Dipper huffed, crossing his arms. "And anyway, aren't you supposed to be working?" "Aren't you?" "Ha! Nice try. Now get to work!"

Dipper sighed, getting up and heading to the front of the shack. Mabel was sitting at the register, looking bored. She perked up when she saw Dipper. "Oh hey brother that I love more than anyone ever! Would you be willing to take my shift just this once in repayment for my undying love to you?" Mabel said, looking hopeful. "Sorry, not happening." Dipper said, grinning. "Hmph. Ass." Mabel said, sticking out her tongue. "Hey, what happened to the whole 'undying love and affection' thing?" Dipper said jokingly.

"Oh, hey Wendy." Wendy waved. "'Sup." She walking over to the register, and her and Mabel traded places. Mabel put on a fake disappointed face. "Oh dear! It seems that my job has been taken! It looks like I have no choice but to go and meet up with my friends. Such a shame!" Dipper scoffed. "Yeah right, slacker. The Shack isn't gonna dust itself, so I suggest you get on it." "UGH." Mabel flopped dramatically onto the floor while Dipper laughed.

~~TIME SKIP~~

"Phew! Finally done!" Mabel said flopping down on her bed. She looked over at Dipper. "Hey, you okay Dip-dop?" She asked, looking concerned. "Huh? Oh yeah, I'm fine. Just a little tired." "Yeah, you look like you are." Dipper yawned. "I think I'll take a nap." Mabel put on a mock shocked face. "What!? My bro-bro going to bed willingly!? Next you'll say you're going to take a shower!" "Ha ha." Dipper said. "And I don't smell that bad!" Mabel raised an eyebrow. "Uh-hu. When's the last time you showered?" " . . . " "That's what I thought." Dipper sighed. "Whatever, I'm gonna sleep now." "K. Later Dipping sauce!" Mabel giggled as she shut the door, hearing a muffled, "Don't call me that!" before skipping down stairs. "

Hey Grunkle Stan!" She said, skipping into the living room. "I'm kinda hungry sooo . . ." Stan looked over at her. "If you're expecting me to make dinner then you might as well just skip it." He said, turning his attention back to the television. "Ugh! Why must everyone make me work!" Mabel exclaimed, slouching into the kitchen.

She decided to make something for Dipper fr when he woke up. She was walking back out to the tv room, when she suddenly felt dizzy. And not the "Stood up too fast" kind of dizzy, but full on "the room is spinning" dizzy. She staggered. Stan looked over, alarmed. "Hey kid, you okay? Mabel!" That was the last thing she heard before falling to the ground.


	3. Chapter 3

" . . . Abel . . .Mabel! Mabel wake up!"

Mable groaned and opened her eyes to see Stan standing (Heh, Stan standing) over her, looking worried. "Ugh, what happened?" "I'm not sure. One second you look fine, and then the next, you're pale as a ghost and look ready to pass out!" Mabel shook her head to get rid of any remaining grogginess, and tried to stand, only to find she couldn't. And was it just her, or was it getting harder to breathe?

"Grunkle Stan?" Mabel said, looking over at him. "I can't get up. Help me?" She assumed she was just weak because, well, she _had_ just passed out! She rethought that though, when she saw Stan flinch. "Grunkle Stan? What's wrong?" She saw his eyes flicker to her legs. Fearing the worst, she looked down. She was _not_ expecting what she saw.

"AAAAAAAAAAH!"

Dipper jerked awake. That sounded like Mabel! Was she okay!? He tried to stand, only to stumble and fall. His legs felt really weird. Something wasn't right here. Maybe his legs fell asleep? He glanced down at his legs. Well, everything seemed normal- WAIT WHERE THOSE HOOVES!? He quickly looked down again.

Stan sighed as he heard a scream similar to Mabel's from upstairs. "Mabel sweetie, just calm down!" Mabel was flopping around on the floor panicking. "I WILL NOT CALM DOWN STAN I AM A MERMAID WHAT IS GOING ON." "I don't know!" Stan said waving his arms around. "But the sooner you calm down, the sooner we can figure this all out!"

He whipped around after hearing multiple thuds followed by and "Ow!" As whoever fell down the stairs reached to bottom. His jaw dropped. 'You have got to be kidding me.' He thought, staring at the now half-deer Dipper. Glancing back at Mabel, he saw she was starting to turn blue. Panicking, he quickly picked her up and ran her to the bathtub, turning on the water. She took a deep breath, letting the water fill her lungs, before breathing out.

"I LIVE!" She said dramatically, flopping back in the tub and splashing water everywhere. "Dipper! Get up here!" Stan yelled, and heard a groan before hearing careful footsteps (hoof-steps?) coming up the stairs. Dipper poked his head in. "going up stairs with hooves is _not fun._ " He said, before walking (um, trotting?) Into the bathroom. "Dipper! I'm a mermaid! How cool is that!" Stan frowned.

"Wait. You were freaking out about this about two minutes ago!" Mabel shrugged. "Well, yeah. But then I realized how cool this is!" Dipper sighed. "Well, I'm glad at least one of us is enjoying this." Mabel looked over at her brother, and her eyes widend. "Oh my gosh bro-bro! You look adorable!" Dipper frowned, blushing. "I'd deny that, but looking like this I really can't argue." Stan looked over at the two.

"So, do either of you have any idea as to what caused this?" Dipper and Mabel shook their heads. "There might be something in the journal though. I'll go get it." Dipper walked out of the room, before returning, journal in hand. He sat down, (Well actually more laid) and started flipping through the pages. He continued flipping through until something caught his eye. "Hey, Grunkle Stan?" Dipper said. "Didn't the paper mention that a dam had broken recently?"

Stan looked over, trying to peek at the journal. "Yeah, why?" "You might want to look at this . . ." He held up the journal showing a page about a river.

" _Today I found something extraordinary! A river that is able to alter a being physical form! I've decided on calling it "The Fluvius Cantatis" or "Enchanted River". I haven't come into contact with any of the water, but from observing the behavior of beings that have, I have figured out that any and all sentient living beings that come into contact with it will be changed. I have yet to find any type of cure for this, and as such, this river should be avoided at all costs."_

Stan stared at the page, slowly comprehending what he had just read. "So . . . there's . . .No cure, for this?" He said slowly. Dipper nodded grimly, before his eyes widened as his great uncle fell to the floor in a dead faint.

"Grunkle Stan!"

 **(A/N)**

 **Hey guys! I just want to thank all the people who reviewed and favourited my story! I'm glad that you like it! :D Also, I won't really have a regular updating schedule. It will be more of an "Update when I can" Kind of thing.**

 **Peace!~**


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